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Trees: reducing their size safely

When a tree outgrows its space, gardeners must decide if it is worth reducing it in size. This is usually achieved through pruning, but it can be hard work and expensive. The alternative is to replace the tree with one to suit the space, but this is not always practical.

Quick facts

Suitable for All trees that tolerate being prunedTiming Winter when dormant, or late summer, depending on the treeDifficulty Moderate to difficult

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Suitable for...

When a tree has outgrown its space in the garden, it will need to be reduced in size. It is better to do this sooner, rather than later, as the longer it is left, the harder it is to prune and less likely to recover.

Trees also might need to be pruned and therefore reduced in size if they have dead, diseased, crossing or torn branches.

It is usually best to try and keep tree (and shrub) growth under control with regular pruning, but this is not always practical, especially if you inherit overgrown trees and shrubs in a new garden.

When to reduce trees

Many trees are best pruned in late summer when healing is most rapid: magnolias and walnuts for example. Prunus (cherry) trees are best pruned in mid-summer to reduce the risk of silver leaf disease.

For most other deciduous trees winter pruning is easier as the branches are more visible but is best accomplished before Christmas as later pruning can result in bleeding.

How to reduce tree size

Depending on the tree involved and the intended effect there are several tree pruning strategies. They are listed here from those requiring the least work, to the most work.

An all over trim in spring or summer: This is only really appropriate for some smaller formal trees, especially evergreens. This will need doing every year or two. For these smaller trees a long-handled hedge trimmer is a labour-saving option.

Pruning when dormant: Usually involves shortening side-branches all over the tree to make it smaller and more attractive. It lets in light and reduces the tree's vulnerability to wind damage and is an opportunity to remove diseased or damaged wood. To get a balanced result, work slowly and carefully and evaluate the effect of removing each branch. This is not suitable for trees prone to silver leaf.

Pollarding: In this extreme form of pruning, the entire head or crown is removed which can make most attractive small trees, although at the price of repeated pruning.

Crown lifting: Lifting the crown by removing lower branches will allow access for mowing, mulching and enjoying the shade cast by the tree.

Crown thinning: Thinning crowns to let in more light by removing some, usually up to 30 percent, of the branches and concentrating on dead or congested shoots is another strategy.It is very easy to spoil the appearance of the tree so this is best attempted in stages evaluating the effect before removing more.

If branches larger than the diameter of your wrist need to be removed or if there is a lot of work up ladders needed, it would be best to call in a professional arborist. Pollarding, crown lifting and crown thinning are also all best undertaken by a professional.

Also be aware if there is a Tree Preservation Orders (TPO) on the tree. The tree officer of your local council will be able to advise if the tree is covered by a TPO.

Problems

A very common error is to give an all over trim without regard to cutting side-branches which results in vigorous tufts of unsightly regrowth spoiling the shape of the tree. These will later need to be thinned out allowing selected shoots to regrow and restore an attractive shape to the tree.

If too much material is removed in once year, vigorous growth can result. This often occurs when apples are over-zealously pruned.

RHS tree experts no longer recommend pruning or wound paints in most cases as they appear to inhibit healing and may actually encourage rotting. To encourage healing make pruning cuts through the branch ‘collar’ (slight swelling where the branch joins the trunk) as this is where natural healing most readily takes place.

Discuss this

I have a70+ year old ash tree which I pollarded in December. 3rd pollard in 24 years. It has a lot of ivy on it up to the top of the pollarded branches. There currently appears to be no new growth emerging which is worrying as it is a majestic tree. Any advice or comments would be appreciated

I have a mango tree that has grown so big over our fence already. Some of its branches are over the road. Last month we trimmed them out because it also disruped the power line to our house. So our house's electricity is not stable, we wondered why. Until our neighbors told us to check if the cables are disrupted. It happened to them as well. Be careful guys.
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